A solar cell, supercapacitor, and a new ultra low power 555 chip make it possible !
Step 2: The CSS555 Micropower Timer IC
The heart of this novelty is the CSS555 timer integrated circuit. It is a micropower version of the venerable 555 and 7555 chips that the electronics world knows and loves. It has the exact same pin configuration and functions the same way. It actually can be user-programmed to work in various modes, but for the project described here, it is used in the standard 555 mode as it comes from the factory. Its key feature for the paperweight is the tiny amount of power that it needs to function. In this application, the circuit takes only about 6uA to carry out its timing tasks. The biggest current draw occurs when it blinks the LED which is only about 1/2% of the time. Thus on average, the circuit uses a tiny amount of power. This enables the paperweight to blink all night long on the energy it stores in its supercapacitor via a solar cell during the day or under your desk lamp.
Step 3: Astable Operation
The basic circuit used here is the standard 555 astable setup. At the start of an ongoing cycle, the timing capacitor CT starts charging up through resistors RA and RB; the discharge pin is open and the output pin is high (i.e. at the supply voltage VDD). When the rising capacitor voltage reaches the upper trip point, which is 2/3 of the supply voltage VDD , the output and discharge pins go low, i.e. are connected to ground or 0V. The capacitor voltage then drops via resistor RB until the lower trip voltage, 1/3VDD, is reached which brings the trigger pin to the lower trip voltage and that causes the output pin to go high again and the discharge pin to disconnect. The cycle then repeats itself as before. Note that the reset pin must be connected high for operation; setting this pin low stops the timer and the output pin goes low.